1. Field of the Invention
This invention relates to a method for growing single crystal III-V compound semiconductor layers and, in particular, single crystal III-V compound semiconductor layers of nitrides.
2. Related Art
In recent years, attention has been directed to III-V compound semiconductors of nitrides such as GaN, AlGaN and GaInN because they have forbidden band widths ranging from 1.8 eV to 6.2 eV and theoretically promise light emitting devices capable of emitting red light to ultraviolet rays.
A light emitting diode (LED) or a semiconductor laser using III-V compound semiconductors of nitrides is made by stacking a plurality of layers of AlGaN, GaInN, GaN, etc. into a multilayered structure. The multilayered structure requires that respective layers have good crystalline properties and good surface conditions.
However, it is difficult to grow a single crystal layer of a desired thickness with no defect because the lattice constants of AlGaN, GaInN, GaN and other III-V compound semiconductors of nitrides are largely different from each other unlike AlGaAs/GaAs semiconductors. In other words, when the thickness of growth exceeds a given value, there is no way to prevent defects produced in the crystal and deterioration in the crystalline property and the surface conditions. Taking an example where an AlN layer is grown on a GaN layer, since lattice constants of AlN and GaN in a-axis directions are different by 3%, the theoretical thickness of single crystal that can be grown without defects is only about 20 nm, and any larger thickness accompanies defects in the crystal and results in deterioration of the crystalline property and the surface condition.
This issue is reviewed taking a practical example where an Al.sub.0.15 Ga.sub.0.85 N layer was grown on a GaN layer. Observing its surface through an optical microscope, a cross-hatched pattern was recognized on the Al.sub.0.15 Ga.sub.0.85 N layer stacked up to a thickness around 300 nm. This aspect is shown in FIG. 2B. The cross-hatched pattern is made by cracks in the Al.sub.0.15 Ga.sub.0.85 N layer caused by tensile forces applied thereto due to a difference between lattice constants of the GaN layer and the Al.sub.0.15 Ga.sub.0.85 N layer. Cracks in grown layers in a light emitting device having a waveguide structure are a cause of optical reflection and deterioration of the device.